Failures in multi-wire harnesses and cable assemblies most commonly are in the form of shorts or opens at or close to one of the connectors. It is advantageous to know at which connector the problem is located.
Shorts can be rather easily localized with the aid of a multimeter or equivalent, since the shortened end of that wired assembly will exhibit a lower pin-to-pin resistance than the read at the corresponding pins of another connector. Identifying which end of a conductor is open-circuited presents more of a challenge.
A viable approach to locating opens is to make use of the fact that wires bundled together will have measurable capacitance to one another. Experience has shown that interwire capacitance may be 30-300 pf. or more in a typical wired assembly. An open pin will have capacitance to its neighbors an order of magnitude lower, for example, 10 pf.
A capacitance meter rather reliably can distinguish open pins from wired ones, and hence could be used to determine which end of an open wire has broken away from its pin. A capacitance meter is relatively expensive, or might not be readily available when needed, regardless of cost. Furthermore, the time and "eyes off" distraction of reading such an instrument could be most inconvenient.